1. Statement of the Technical Field
The inventive arrangements relate to wireless networks, and more particularly to wireless networks which communicate in accordance with a frequency hopping scheme.
2. Description of the Related Art
Many wireless communications systems use so-called frequency hopping methods to allow communication among nodes in a manner which makes use of a number of communications channels (frequency bands), one at a time, and varying the radio frequency channel in time according to a pre-determined sequence. Usually this channel “hopping” occurs several times (e.g., 3-5 times) per second in accordance with a very regular schedule. The time spent on each radio frequency or channel is defined as the “dwell time” or “dwell time duration” and the extent of this time period is usually held to a constant value.
The value in frequency hopping resides in the fact that (1) it allows systems to spend a minimum of time on any one channel or small number of channels which may be disturbed by electromagnetic noise in the environment, (2) it allows multiple systems to coexist and use a common set of channels, and (3) it provides for additional security, apart from encryption of communicated data. However, in certain types of wireless networks, the nodes which comprise the network sleep for the vast majority of time and have a great difficulty keeping an accurate time clock. An example of such a wireless network is a low-cost low-duty-cycle wireless sensor network. In such networks one great challenge in implementing frequency hopping is providing some mechanism or method by which the various nodes (which are sleeping or inactive much of the time) can maintain channel hop synchronization with other nodes in the network. This problem is made especially difficult where such synchronization is to be maintained without regular messaging among the nodes to achieve timing correction.
There are two common solutions to the problem of maintaining hop synchronization in wireless networks as described herein. According to one approach, each wireless node participating in the network uses a real-time clock and is required to wake up often enough to maintain the accuracy of that clock. The problem with this approach is that the frequency at which the node must wake up for purposes of maintaining synchronization may be so great that it uses over time a significant amount of the limited power resources available at the node. Accordingly, this approach can greatly reduce the battery life of the wireless nodes in the network. An alternative approach to the problem accepts the fact that an inactive or sleeping node will not maintain synchronization. Instead, when a particular node needs to communicate it first scans the network to “re-find” the currently active channel of the network. But this network scanning process can also consume a great deal of power and thus reduce battery life of the battery powered nodes in the network.